The invention relates to a series wound commutator motor suitable for an electric power tool comprising switching means for switching between a motor operation mode and a braking operation mode, wherein in motor operation, at least a field coil and an armature winding are connected in series in a motor circuit fed with a voltage supply, and wherein in braking operation, the at least one field coil with the armature coil form a closed braking circuit separated from the voltage supply.
Such a series wound motor is disclosed in EP 0 471 038 B1. The known motor is particularly suited for driving inverse-speed electric power tools, such as right-angle grinders or circular saws and employs a multi-pole switching unit for switching between motor and braking operation modes. The switching unit short-circuits the motor in braking operation and the pole of the field coil is reversed. Means are also provided for limiting the braking current through the field coil. In this manner a smooth and rapid short-circuit braking through a separate auto-excitation is achieved. The armature or rotor is connected between the field coil and the commutating winding in motor operation, while in braking operation a current path is provided between the armature and the commutating winding. The current path comprises a Zener diode arrangement for limiting the braking current, such that only a predetermined amount of the braking current flows across the field coil.
In such motors, a reliable braking or retardation of the motor by switching into the braking operation is achieved, however, the use of commutating windings is more complicated and expensive.
Attempts have been made to avoid such problems by providing a capacitor to be charged in motor operation, which is then used for braking by auto-excitation (DE 42 01 023 A1). The known circuitry has the disadvantage that the stored charge can only be used once for introducing braking. If the charge is not sufficient to initiate braking, then braking cannot be expected to occur after capacitor discharge.
An object of the present invention is therefore to provide an improved series wound motor, which avoids the above disadvantages, in particular which is simple and inexpensive in structure and which guarantees a reliable braking when switching to braking operation.